The generic type T is not declared anywhere. This causes exceptions on OpenJDK such as:

java.lang.NullPointerException
at com.sun.beans.TypeResolver.resolve(TypeResolver.java:321)
at com.sun.beans.TypeResolver.resolve(TypeResolver.java:351)
at com.sun.beans.TypeResolver.resolve(TypeResolver.java:310)
at com.sun.beans.TypeResolver.resolve(TypeResolver.java:157)
at com.sun.beans.TypeResolver.resolveInClass(TypeResolver.java:78)
at java.beans.FeatureDescriptor.getReturnType(FeatureDescriptor.java:368)
at java.beans.Introspector.getTargetEventInfo(Introspector.java:1020)
at java.beans.Introspector.getBeanInfo(Introspector.java:424)
at java.beans.Introspector.getBeanInfo(Introspector.java:189)
at grails.test.mixin.web.ControllerUnitTestMixin.mockController(ControllerUnitTestMixin.groovy:268)